LECTURE IX. 



ALBUMINS OR PROTEINS. 



III. 

 COMPOSITION OF INDIVIDUAL PROTEINS. CONSTITUTION. 



IN the formation of proteins, the amino acids alone participate, as far 

 as we know, with the single exception of the amino-hexose, glucosamine, 

 which occurs in many varieties of albumin. The number of these amino 

 acids already discovered is very large. It includes the following : glycocoll, 

 alanine, amino-valeric acid, leucine, isoleucine, a-pyrrolidine-carboxylic 

 acid (proline), oxypyrrolidine-carboxylic acid (oxyproline), serine, phenyl- 

 alanine, glutamic acid, aspartic acid, tyrosine, cystine, tryptophane, lysine, 

 histidine, arginine, and diaminotrihydroxydodecoic acid. It is of chief 

 interest to learn whether the proteins, at present known, contain the same 

 fundamental substances, or whether specific groups of proteins are char- 

 acterized by their content of individual amino acids. Another matter of 

 considerable importance is the relative quantity of the different amino 

 acids, occurring in the proteins. It is possible that the differences between 

 the various proteins are due to varying relations of the quantities of 

 individual amino acids present. On the other hand, it is a matter of the 

 greatest importance to know the quantitative amounts of these con- 

 stituents of the albumins for use in furthe? work on this subject. We 

 should like to know how great a portion of the whole albumin molecule 

 is already understood. Unfortunately, we have no quantitative method 

 for estimating the amino acids. True, we can estimate very exactly some 

 of the cleavage-products, like tyrosine and glutamic acid, but for the 

 remainder of the amino acids we can only estimate the approximate 

 amounts. Our knowledge concerning protein formation was, until re- 

 cently, very limited. 



Although various amino acids had been isolated, and the quantitative 

 relations of lysine, arginine, and histidine in different albumin molecules 

 had been established, through the researches of A. Kossel, investigators as 

 a rule attempted only to prepare the proteins in as pure a form as possible, 

 and to classify them according to their elementary composition. A turn- 

 ing-point in the whole chemistry of the albumins was reached when E.- 

 Fischer l introduced a new method for isolating the amino acids. Briefly, 

 the process consists in forming the esters of the mono-amino acids, and 



1 E. Fischer: Z. physiol. Chem. 33, 151 (1901). 



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